The present invention relates to a color cathode ray tube used for a color image display, and particularly to a color cathode ray tube accommodating an in-line type electron gun for emitting three electron beams arranged in the horizontal direction of a screen with improvement in the shapes of electron beam apertures in plate electrodes forming a main lens of the electron gun.
A color cathode ray tube used for TV picture tubes and information terminal monitor tubes accommodates an electron gun for emitting a plurality (in general, three) of electron beams in an evacuated envelope at one end thereof, a phosphor screen composed of phosphor films of a plurality (in general, three) of colors coated at the opposite inner end of the evacuated envelope, and a shadow mask serving as a color selection electrode disposed closely spaced from the phosphor screen toward the electron gun. A desired image is displayed by two-dimensionally scanning a plurality of electron beams emitted from the electron gun by magnetic fields produced by a deflection yoke disposed outside the evacuated envelope.
FIG. 15 is a sectional view illustrating a configuration example of a color cathode ray tube to which the present invention is applied. Reference numeral 21 indicates a panel portion; 22 is a funnel portion; 23 is a neck portion; 24 is a phosphor film; 25 is a shadow mask; 26 is a mask frame; 27 is a magnetic shield; 28 is a shadow mask suspension mechanism; 29 is an in-line type electron gun; 30 is a deflection yoke; 31 is a beam adjustment device; 32 is an internal conductive coating; 33 is a tension band; 34 is a stem pin; and 35 is a getter.
In this color cathode ray tube, an evacuated envelope is composed of the panel portion 21, the neck portion 23, and a funnel portion 22 connecting the panel portion 21 to the neck portion 23.
The panel portion 21 has on the inner surface thereof a display screen composed of the coated phosphor films 24 of three colors, and the neck portion 23 accommodates the electron gun 29 for projecting three electron beams in line. The shadow mask 25 having a multiplicity of apertures or a parallel array of narrow strips held together only at the ends is disposed closely spaced from the phosphor film 24 of the panel portion 21.
Reference characters Bc and Bs indicate electron beams, and the deflection yoke 30 is mounted externally of the transition region between the funnel portion 22 and the neck portion 23.
The getter 35 is supported by the extended end of a getter support spring which is fixed at the other end thereof to a shield cup of the electron gun 29 by welding in the electron gun assembling process. The getter 35 increases the degree of vacuum in the evacuated envelope by evaporating and scattering a getter material within the evacuated envelope.
Three electron beams emitted from the electron gun 29 are deflected in the horizontal and vertical directions by vertical and horizontal deflection magnetic fields produced by the deflection yoke 30, after being subjected to color selection through electron beam apertures in the shadow mask 25, and strike intended phosphors, to form a color image on the phosphor film 24.
FIG. 16 is a schematic sectional view illustrating a configuration example of an in-line type electron gun accommodated in a color cathode ray tube. Reference numeral 1 indicates a cathode; 2 is a first grid; 3 is a second grid; 4 is a third grid; 5 is a fourth grid; and 6 is a shield cup.
In this electron gun, the first grid 2, second grid 3, third grid 4 and fourth grid 5 are arranged in this order from the cathode 1 side, with the shield cup 6 connected to the fourth grid 5, to form the so-called "bipotential type lens". A main lens is formed between the low-voltage third grid 4 and the high-voltage fourth grid 5.
The third grid 4 and the fourth grid 5 are composed of tube-like electrodes each having a transverse cross section with its major axis in the in-line direction, that is, in the horizontal direction, and contain plate electrodes 41 and 51 disposed at positions set back from the opposing ends of the tube-like electrodes, respectively.
FIGS. 17A and 17B are front views of the main lens portion taken along line XVII--XVII, looking toward the cathode 1, in FIG. 16. FIGS. 17A and 17B show separate configuration examples of the plate electrode 41. FIGS. 18A and 18B are front views of the main lens portion taken along line XVIII--XVIII, looking toward the shield cup 6, in FIG. 16. FIGS. 18A and 18B show separate configuration examples of the plate electrode 51.
In FIGS. 17A, 17B, 18A and 18B, reference characters 4s and 5s indicate electron beam apertures for side electron beams, and 4c and 5c indicate electron beam apertures for a center electron beam.
Each of the third grid 4 and the fourth grid 5 is formed of a tube having a non-circular transverse cross-section whose diameter is larger in one direction (the in-line direction, that is, the horizontal direction) than in the other direction (the vertical direction). The plate electrodes 41 and 51 are disposed in the tube-like electrodes 4 and 5, respectively.
The in-line type electron gun having the configuration shown in FIGS. 16, 17A, 17B, 18A and 18B operates as follows.
Thermoelectrons emitted from the three cathodes 1 heated by the corresponding heaters positioned in the cathodes 1 are attracted toward the first grid 2 by a positive voltage applied to the second grid 3, to form three electron beams Bc, Bs, and Bs.
The three electron beams Bc, Bs and Bs pass through the electron beam apertures in the first grid 2 and through the electron beam apertures in the second grid 3, and are accelerated and focused by the main lens formed between the third grid 4 and the fourth grid 5 to be directed toward the screen.
The plate electrodes 41 and 51 disposed in the third grid 4 and the fourth grid 5 forming the main lens are supplied with a low voltage of from 5 to 10 kV which is the same as that applied to the third grid 4 and a high voltage of about 20 to about 30 kV which is the same as that applied to the fourth grid 5, respectively.
Since an electron lens formed between the side electron beam apertures 4s in the third grid 4 and the side electron beam apertures 5s in the fourth grid 5 is non-axially-symmetric, the path of the side electron beams Bs is bent toward the tube axis by the main lens. As a result, there can be obtained static convergence for converging three electron beams at the screen center.
In the prior art in-line type electron gun having the above configuration, each of the third grid 4 and the fourth grid 5 usually has an opening of a non-circular transverse cross section in which the horizontal diameter is larger than the vertical diameter.
In the case where the main lens is formed only by the opposed third grid 4 and fourth grid 5 each having a substantially elliptic cross-section, the focus characteristic differs between the vertical and horizontal directions because the major diameter (horizontal diameter) of the cross-section of the opening in each grid is different from the minor diameter (vertical diameter) thereof and in general a large difference is set between the major diameter and minor diameter. This causes a problem that the beam spots on the screen are horizontally elongated.
To solve such a problem, as in an electron gun disclosed in Japanese Patent Laid-open No. Sho 58-103752, each of the electron beam apertures formed in the plate electrodes 41 and 51 respectively disposed in the third grid 4 and the fourth grid 5 is formed in a substantially elliptic shape having a minor diameter in the horizontal direction for suppressing the elongation of the electron beam spots in the horizontal direction.
Namely, electron beams passing through the electron beam apertures in the plate electrodes are focused stronger in the horizontal direction than in the vertical direction, and are relatively rounded in cross-section, to thereby solve the problem that the beam spots on the screen are horizontally elongated.
In the prior art in-line type electron gun, however, the inner wall of the tube-like electrode is close to side electron beams and thereby the effective lens diameter is small, and consequently a voltage for optimum focus becomes higher. This causes a problem that there easily occurs a difference between the optimum focus voltages necessary for the center electron beam and the side electron beams.
Consequently, when the focus voltage is adjusted for optimum focus of one of the center electron beam and the side electron beams, the other of the electron beams is out of optimum focus and thereby the beam spots on the screen are enlarged.
In the prior art in-line type electron gun having the above problem, when the positions of the above plate electrodes are modified for equalization of the focus voltages for the three electron beams, there occurs adverse effect of disturbing static convergence of side electron beams.
To cope with such a problem, there has been proposed a method for equalizing the focus characteristics for three electron beams by making the major diameter of an electron beam aperture for the center electron beam different between a low voltage plate electrode and a high voltage plate electrode. In this method, however, the major diameter of the center electron beam aperture in one of the plate electrodes must be reduced, causing a problem that the main lens diameter is reduced.